


Buoyant Bones and Shining Scales

by Vienamarie



Category: Undertale (Video Game)
Genre: F/M, Fluff and Smut, Friends to Lovers, Swimming
Language: English
Status: Completed
Published: 2016-12-21
Updated: 2017-01-03
Packaged: 2018-09-10 23:02:26
Rating: Mature
Warnings: No Archive Warnings Apply
Chapters: 2
Words: 13,914
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/8943025
Author URL: https://archiveofourown.org/users/Vienamarie/pseuds/Vienamarie
Summary: Sans teaches Undyne to skip stones. Undyne teaches Sans to swim.





	1. Chapter 1

“Not bad, not bad,” Undyne smirked. She watched Sans skip a stone several times before it dipped under the water. Onion-San, enthralled to play, dove under the surface to retrieve what the two of them had skipped so far, leaving them by themselves for a moment. “You’re stronger in your arms than I gave you credit for, punk.”

“You do know,” the skeleton hummed, picking up another rock, “that the force of the throw means nothing, right? It’s in the momentum of your wrist when you flick it.”

“Yeah, yeah, science-y shit. Leave that to Alphys, okay?” she said, waving her hand dismissively. She picked up a stone of her own and chucked it as hard as she possibly could at the water. While it certainly traveled farther than any of Sans’, it only skipped once before plopping in. She resisted scowling and pointedly ignored the look he shot her way after that. 

“…okay, so maybe teach me some of that science-y shit after all.”

Sans laughed. The sound pleased Undyne as it reached her fins, and she sat on her knees so he could effectively reach and guide her arm through the motions of tossing a stone. They paused, briefly, because Onion-san popped up to deliver a pile of rocks to them before giggling and diving back under the surface of the lake.

“You can swing your arm as hard as you want, but all you’re really supposed to focus on is your hand and wrist…” Sans explained, his voice calm and deep next to the warrior woman’s head. She wasn’t flustered – Royal Guards didn’t GET flustered!!!! – listening to the skeleton talk to her, or giddy from the gentleness of his fingers covering her own, guiding her slowly from beginning to end. “Swish, and flick…swish…and flick. Give it a try.”

Her expression, calm, shifted to one of determination. She stood back up and flexed with a grin, pleased in the way Sans watched her muscles move and ripple. Then, with a stone in hand, she did as he taught her and flicked it flat across the surface. 

It skipped one…

Two…

Three…

…six times across the water! Even Onion-San was impressed, cheering them on from across his lake before diving in after it, tentacles swaying and making gentle waves. 

“Good job,” Sans praised, turning to the warrior woman. The lights in his sockets were bright, clearly happy. “You’re a quick learner.”

“Heh, ‘course I am!” She said, punching her palm. “I CRUSH CHALLENGES INTO POWDER AND MIX THEM INTO MY PROTEIN SHAKES!!”

“You could really say that…”

Undyne rolled her good eye, dropping her hands at her sides. “Here we go.”

“ _…you devour the competition._ ”

The Captain of the Royal Guard glared at her companion, crossing her arms. “That was AWFUL!”

Sans shrugged. “Can’t win ‘em all. You wanna skip s’more?”

The woman shook her head, grasping his wrist as he bent to pick up another rock. There was a light in her eyes he couldn’t place. “Nah, now that I can do it, it’s boring! Let’s swim instead!”

“I love swimming,” Onion-San called, moving closer to the two of them. “You can swim with me!”

“Ah, ‘fraid not,” Sans said, his smile apologetic as he looked to the cephalopodic monster. “The water would just go _right through me_ , big guy.”

Undyne groaned.

“I just wasn’t made for the water.” Gently, he pried his hand away and stuffed it in the pocket of his cyan hoodie. “I’m more than content to watch, though. Go ahead, Undyne.”

Undyne smirked, then, nudging him. “Bet you are, perv. I’ll teach ya’ how to swim!”

She reached over and grabbed his hand again, pulling it out of his pocket and threading her webbed fingers through his boney phalanges. The odd sensation wasn’t lost on either of them as she began to power walk away, bidding Onion-San goodbye. He waved a tentacle after them. “Come play again! I’ll be here!”

Sans, though he didn’t physically protest, voiced his concerns.

“Uh, Undyne?” he said. She simply grunted in acknowledgement. “I dunno if you were paying attention before, but, er, I’m not able to swim.”

“And five minutes ago I couldn’t skip a stone. I WON’T STAND FOR QUITTER TALK!!!” She barked good-naturedly at her friend, walking faster. Due to their height difference and linked hands, Sans was forced into a light jog after her. “I couldn’t swim when I was a guppy, punk! You think that stopped me? Everybody can learn to do things!”

“I think you’re missing my whole _point_ ,” the skeleton tried again, poking her hand with the tip of his finger. She rolled her eye and kept speed. “My body, physically, is too dense for water. I’ll sink like the rocks we skipped.”

“Then I’ll hold ya,” she reasoned. Knowing that she was too stubborn to back down from this one, Sans merely let her tug him along, resigned to spending the rest of his afternoon stuck at the bottom of a lake.

The two of them usually spent their outings like this, when they weren’t working or Undyne wasn’t giving Papyrus cooking lessons. One of them would choose an activity (Sans almost always suggested napping and Undyne almost always suggested suplexing boulders), then block out the rest of their day to hang out. Even if the activity itself didn’t take very long, they dragged it out and wasted the day away, regardless.

A year ago, Sans realized Undyne wasn’t just a screaming, power-hungry fish person, and Undyne realized that Sans wasn’t just a narcoleptic, introverted bag of bones. They’d pushed past their initial impressions for Papyrus’ sake, because he wanted to be able to hang out with both of them at the same time, and now they were almost never apart on their days off. The taller skeleton wasn’t getting quite what he wanted, but he was perfectly fine with it, stating that it was “GOOD TO SEE MY BROTHER BEING LESS OF A LAZYBONES AND MY COOKING BUDDY BEING LESS OF A CULINARY ARSONIST.”

Undyne liked Sans. Sometimes, only sometimes, his jokes were funny, and he was much stronger than he liked to let on. Sure, he wasn’t as physically fit as Papyrus, but he got around when he wanted to and didn’t complain when she tugged him along. 

By the same token, Sans liked Undyne. Sometimes she was gentle, and she was much smarter than, apologetically, he gave her credit for. Sure, she dubbed everything that involved mathematics and complex magic “boring, science-y shit,” but she was willing to sit and learn something she might not’ve known before.

She was also quick to introduce him to new things, which is why they were now standing at the edge of a smaller lake in the middle of Waterfall. It was shallower near where they were positioned, but farther out it got deeper, past where Sans could comfortably stand above the surface. 

Undyne released his hand to shrug her shirt off, leaving the cyan monster in just a sports bra and the shorts she wore out of her home. Turning to him, she frowned to see he hadn’t begun to remove any of his clothing.

“C’mon, punk! Start stripping - BEFORE I DO IT FOR YOU!!”

“That’s kinda forward, isn’t it?” Sans smirked up at her. “Take me to dinner first, Undyne, geez. I’m a skeleton with class.”

“NNNGAAHHH!!” She roared, grasping him by the jacket and forcefully yanking the zipper down. In her excitement, the woman missed the way his breath hitched from the action. She pulled it off him and threw it down next to her shirt, leaving him in a white tank top and his own basketball shorts. “SHOES, OFF!!”

“You’re not gonna do that for me, too?” He teased, but complied nonetheless. Sans often found it easier just to go along with what she wanted, lest he come home with torn articles of clothing and slightly worse hearing. Undyne enthusiastically yanked her own boots off and dropped them carelessly to the side, their clothes now in a small pile. “Just, don’t be disappointed when you finally figure out that I stand no chance of floating.”

Her smile was smug, and she chose to say nothing as she stepped into the water. Sans didn’t automatically move to step in after her, so she took him by the hand again and tugged him in. Slowly, they waded towards the deeper end of the lake, until he was submerged up to his sternum.

“Do you need to breathe, Sans?” She asked. Blinking, the skeleton looked up. 

“…I, uh, can’t drown, no,” he said, and felt his soul lurch at the way her smile turned feral. “Why, what –“

“START SWIMMING, PUNK!!!” She cried, lifting him under his arms and chucking him into the water. He cried out in surprise, silenced quickly as his form plunged under the surface. 

For several seconds, nothing happened, and Undyne watched the water in silence. Her confidence began to waver, then, turning to doubt, then panic.

“Sans?” she called, moving farther out until she was swimming. She began glancing all around her in frantic spins. Had it not worked? Had she done it wrong? When no response came, she prepared to dive down and retrieve him. Hopefully, she’d be able to reach the bottom. “Sans!”

About to submerge, she was stopped by a blur of white. Sans surfaced, bobbing, and spat out a bunch of water. Her shoulders slumped in relief, turning instead to amusement.

“W-what…what the hell?” he coughed, inhaling air into his pseudo-lungs. His arms flailed uselessly for a few seconds before Undyne grabbed them, steadying his form. “I can’t…I’m too dense to float, this is physically impossible.”

Ah, so he wasn’t mad she’d chucked him into the water, then. Good. The smile returned to her face as he looked her way.

“Surprise, doofus,” she said, letting him cling to her shoulders as he tried to settle himself. “I did some science!!!”

Confused, the male simply looked at her for answers. She swiped her hand through the water and lifted it up, showing him a handful of tiny, white minerals.

“I visited the Hotland labs and asked Alphys how to make a skeleton float. She said it wasn’t possible.” 

“It isn’t,” Sans said, still thrown. “It shouldn’t be.”

Undyne waved her hand, more pointedly this time. “Shut up and listen. She said a skeleton can’t float in a natural body of water, so I asked her how to make it _unnatural_ , and she gave me, like, a shit ton of salt.”

Sans’ pupils widened in realization. “You made the water denser than me,” he said, “by filling it with enough salt to make sure I’d float?”

“WAY TO SPOIL THE STORY, THEN,” she huffed, splashing him. He sputtered and splashed her back. “Yeah, nerd! It took hours! I had to bring Papyrus out here for a test run to make sure I put enough in!”

Something in the skeleton’s expression changed. Undyne didn’t have time to decipher it before he was staring at her again, more intensely than before. The warrior woman was suddenly hyperaware of their proximity and tried not to be flustered by it. 

“You did this for me?” he asked. Undyne grinned, two razor-sharp rows of teeth on display for him. 

“Heck yeah,” she said, fist pumping the air. “and the look on your face was AWESOME!!” Calming down a little, she placed her hands on his, gently moving them away from her shoulders and holding them in her palms. “I _know_ skeletons can’t float, nerd. I’m not that…” she cringed, looking away, and Sans’ eye lights brightened. “…dense.”

The smile on his face made the grueling task of filling this stupid lake with itchy, irritating salt for hours worth it. His laughter made her want to do it all over again. 

And laugh he did, more surprised than anything that she’d willingly made a pun. He rested his face on her shoulder to muffle the sound, lightly squeezing her hands in mirth. She squeezed back, used to the countless times they’ve exchanged friendly physical contact, and grinned. 

When he’d calmed down, he pulled away, one hand still in hers to keep him steady, and tilted his head.

“You really are a lot smarter than I give you credit for,” he breathed, sounding almost astonished, and it made her heart pound harder. He smiled wider and used his free hand to give her a thumbs up.

“Okay, Undyne,” he said, cadence light and eager, “teach me how to swim.”

 

oOo

 

Neither monster knew how long they’d been in the water. Seeing as Undyne was naturally aquatic and Sans didn’t have skin, the risk of them catching a cold from overexposure was nonexistent.

Lessons were slow-paced. Knowing that the skeleton had never been able to float before, Undyne taught him the basics of keeping himself above the surface before moving to anything harder.

“Just lie on your back.”

“Sure…uh, how?”

\--

“Okay, now do this! It’s called the breast stroke!”

“Yeah, yeah, okay…uh…how?”

\--

“Okay, now we’re gonna do what you _just_ did, but underwater!”

“…uh –“

“NNNNGGGAHHHHH!!!!”

Needless to say, Sans was picked up and thrown across the lake in frustration several times during the lesson. It was all in good fun, though, given that the grin hadn’t left his face the entire time. Undyne felt…pretty good about that, actually. One would expect a skeleton to have a grin always stuck to his face, but Sans’ was almost eerily expressive. He could laugh, scowl, yawn – the works, though his neutral face was a loose smile. 

Sans reemerged from the water and parted his jaw, letting the water evacuate his skull as fast as possible. Undyne snorted at the scene and he shot her a playful glare in response. 

“Cut me some slack, fish woman,” he called, slowly making his way back over to her. She floated leisurely on her back, hands folded over her stomach and hair drifting freely around her head as she watched him swim. “I’m _drowning_ in all this new information.”

“I’ll drown you in somethin’, alright!” she barked at him. “Show me some hussle!”

“You think I’m not trying?” he huffed, movements still stiff and awkward. It was to be expected from someone who was never intended to possess buoyancy. “The water just _goes right through me_. There’s nothing to push against if I’m not solid enough to do so.”

“BOO!!” She responded, straightening. A fiery, red lock drifted over her eye patch and she flicked it away. “You used that one earlier! AND DIDN’T I SAY NO EXCUSES!?!”

“Yeah, two hours ago. Make an exception,” he pleaded, and as he reached her, it was clear that he was exhausted. On a good day, Sans walked, maybe, two miles if you dragged him by the wrist. After all the time they’ve spent in the water, he probably swam the equivalent of ten. Air left him in soft pants, and his sockets were drooping now that he was still. Undyne’s expression softened and she took his hands again. 

“Do you want to wimp out like a baby and head home for the day?” she asked, as nicely as she was capable. Sans shook his head, which pleasantly surprised her. “You’re pooped, punk.”

“Skeletons can’t poop,” he said, then quickly added, “and please don’t ask Alphys to find a way. That’s something I’m really not interested in learning.”

Undyne cackled, lying backwards again and pulling him with her. He ended up sitting on her stomach, straddling her as she backpaddled around the water. The palms of his hands rested softly on her ribs for leverage, staring idly down at her. She stared back up at him, silently raising a brow and poking her tongue out. He smiled and released a breathy chuckle, breaking gazes to stare up instead.

The cavern of Waterfall they were in was littered with glowing crystals, some as low as the bottom of the lake they were in, others stretching as high as the “ceiling” of the Underground. Shades of blue, white, and purple surrounded the two of them and Undyne, eloquently, thought they made Sans’ bones look less like boring printer paper and more like sparkly marble. The water dripping from his body only served to augment her comparison. 

While he was distracted, she let her eye roam. She looked at the roundness of his skull, the subtle definition of his jaw, the prominence of his collarbones. She looked at his tank top, absolutely soaked and clinging to his ribcage in an aesthetically pleasing fashion. She looked at his hands, nicked and worn from years of magic use, resting calmly on her torso. 

Dammit, she had the hots for the stubby skeleton. Though, she knew, smiling uncharacteristically soft at his face as he looked in mild fascination at the crystals, that she’d greatly liked him for a while. 

He made her feel needed every time he wanted to quit a physically taxing activity or refused to wake before noon. He made her feel happy every time she and Papyrus fucked up her stove trying to “cook” and offered to fix it without hesitation. He made her feel attractive every time he complimented her appearance, be it in her Guardsman armor or the shirt and shorts she threw on this morning (that one meant a little more to her than she would ever let on to anyone).

In her musings, Undyne had zoned out, her gaze locked on Sans as he finally looked away from the crystals to speak to her. Moving on autopilot, her arms and legs still paddled the two of them around. Amused, he lifted a hand to wave in front of her face when he realized how close they were to the edge of the cavern.

“Undyne!” Sans gasped, leaning down. His face was inches from hers as one of his hands cradled the back of her head, the other reaching out to push against the cavern wall before she slammed right into it. He sent out a pulse of magic through his left hand and into the wall, using the momentum to counteract their own and slow her down before she could make impact. 

The woman snapped back into it with a flinch, making sheepish eye contact.

“Ah, shucks,” she said, glancing up at the wall. “Wouldn’t be the first time I’ve knocked my head against it. Thanks, man.”

“I can tell,” the skeleton huffed good-naturedly, his hand still cradling her head. His fingers moved through her hair to her scalp, brushing against ridges and scars from years of impact. The fish monster shuddered and he stopped, beginning to withdraw. “Sorry.”

Glad to be cold-blooded, she didn’t blush as she met his gaze. “…I didn’t say stop, punk.”

Sans hesitated, something foreign painting his expression. This close to the edge of the cavern, the crystals made the lights in his sockets look brighter. Then he nodded, almost imperceptibly, and brought his phalanges back to her head. Gently he caressed her wounds, long healed and hidden beneath a mane of crimson hair, and she relaxed beneath him.

Tentatively, as if reaching out to a skittish animal, her hand came up to the back of his skull, brushing tenderly against the bone. Sans’ socket lids…eyelids?...drooped, but he didn’t draw away. Encouraged, she let her fingers dance over his head, feeling faint nicks and scratches embedded in the bone. Finding a slightly deeper one, she traced it and watched him react. 

His eyes shut and he shuddered, the hand in her hair gripping just a little tighter. Undyne’s breath hitched in response.

She lifted her other hand from under the surface, watching water run off of it in rivulets, and cupped Sans’ cheek. His eyes opened a sliver, the lights inside barely visible, and the look he gave her sent shivers up her spine. Both hands moved to his face, cradling it like a valuable treasure, before she began to lean upwards.

The skeleton leaned down in response, but when their mouths were a breath away his eyes widened, and he quickly sat back. The speed of his motion made him slip off her body and he vanished back under the water a moment, resurfacing a foot away. 

“Sans?” She said, confused. They were going to kiss, weren’t they? Why’d he stop? “You okay?”

“…yeah, I’m…” he mumbled, definitely not okay. “I’m fine.” 

Something in Undyne’s face must have told him she didn’t buy the bullshit, because his cheekbones flushed blue and he couldn’t look her in the eyes. 

“This is…” he said, struggling to form words. Undyne had never known him not to have any at the ready. “We should call it a day. Pap’s probably worried.”

He turned, beginning to swim back towards the edge of the lake, but she was the faster swimmer and caught up to him in seconds, blocking the way.

“Hey, hey, what’s the rush? I’ve never seen you actively move that fast for anything except your bed,” she jested poorly, though inside she was panicked. Was Sans put-off by what she’d just attempted to do? Was she reading him wrong? She’d thought, looking at him bathed in crystal light, that he wanted to kiss her just as much as she did. “Talk to me, man.”

She reached for his hands but he pulled away. The action hurt her and regret flashed in his expression, but he didn’t move to correct his mistake. She stiffened, coming to her own conclusion.

“Undyne, I –“

“You can…” she interrupted, feeling a cold that had nothing to do with her blood. “You can reject me verbally, you know. I can take it, Sans. No hard feelings.” That had to be it. He didn’t share the same feelings she did and he didn’t know how to politely let her down. 

“I don’t want to,” he mumbled, almost too soft to be heard. The warrior woman had caught it, though, and her heart began to beat faster. 

“Then,” she said, “why did you pull away?”

He cringed, obviously uncomfortable with the question. “There’s nothing to kiss.”

“…huh?”

“Undyne, there’s nothing to kiss,” he repeated, a frustration in his eyes that wasn’t directed at her. “I have no lips. I have no skin. Hell, I barely have any HP.” 

Undyne blinked the water from her eye, lost. “I don’t follow.”

“That’s not something you want,” he said, looking pensively at her. “You don’t want a skeleton that spends half his time sleeping at work and joking his way out of chores, that only moves when he has to and can’t reach for half the shit in his kitchen without a step stool. Undyne, you don’t want…” he palmed at his face, unusually solemn. “You don’t want me.”

She understood, then, what he was telling her, and felt anger quickly rising to the surface. 

“What, is that it?” she asked, and now it was Sans’ turn to appear confused. “For real?”

“…I wouldn’t lie to you,” he frowned. “I don’t deserve you, Undyne.”

“Who says?!” She demanded. “Who told you that, huh!?”

“What do you mean?”

“WHO TOLD YOU THAT LOAD OF BULL!?!?” She roared, loud enough that the water rippled a little from the soundwaves. Sans was glad, in that moment, that he didn’t have ears to deafen. 

Undyne tore through the water and wrapped her arms around him, making the skeleton yelp in surprise as she began to shake him like a can of soda.

“Listen, dummy,” she snarled, “I’M the only one that gets to decide who does and doesn’t deserve me, GOT IT!?!? AND YOU’RE MEETING ALL MY STANDARDS, SO OBVIOUSLY YOU TOTALLY DESERVE ME!!!!”

She stopped jostling him around and Sans took a moment to get his sight back in order, focusing on the woman that was now very, very close to him and whose fingers were curled in the spaces between his ribs. He shuddered at the sensation, meeting her gaze with hesitance. She grinned and leaned in closer.

“Listen, punk,” she growled into the side of his skull. “I dunno who hurt you enough to make you think that you weren’t worth my time, but if you gimme their names, I’ll fuckin’ wreck ‘em.” His grasp on her arms increased and she knew she hit the mark, drawing back to look him in the eyes. He looked like he wanted to speak but couldn’t form the words.

“Fuck whatever you’re thinking right now; thinking’s for nerds and I’m not a nerd. Listen to this,” she paused, drawing in a deep breath and letting it out. “You’re hot.”

Sans flushed a brighter blue and she smirked, continuing.

“You’re hot and your magic is the fuckin’ bomb. I’ve never seen ANYONE that can do blue magic like you. And you aren’t physically stronger than I am so I don’t gotta flex 24/7 around you like Papyrus. That shit gives me cramps and I know he can’t cramp and it sucks.” She knew she was sort of rambling at this point, but her vocabulary had never been that extensive to begin with and Sans understood the points she was trying to make. “I don’t give a fuck that you sleep half the day away. I don’t. If I need you, I’ll wake you up. I don’t care if you slack off at your sentry post – don’t tell Asgore – because I know that if a human were to pass by then you’ve got ultra kickass magic that can handle them. I don’t care that your short because I secretly always liked carrying you around.”

Sans didn’t meet her eyes. His gaze was on the water, instead, his reflection rippling from their movement. 

“That’s great, Undyne,” he breathed, “but none of that’s gonna change the fact that I’ve got one HP. Anything as simple as a bad cold can due me in.”

“Punk… THAT MATTERS LEAST OF ALL!!!” She shouted at him. “I’LL PROTECT YOU!! I’LL PUNCH ANYBODY THAT TRIES TO HURT YOU!!! I’LL PUNCH A MOLDSMALL!!! I’LL PUNCH AN ECHO FLOWER!!! I’LL PUNCH BACTERIA!!!!!”

Undyne calmed down and collected herself, slightly winded, and pulled him closer until they were chest to chest. 

“Opposites attract. Or something along those lines,” she concluded, almost a whisper. One hand released him to gently trace the rim of his right eye socket, where she knew magic no longer resided, where she knew he was blind. “Besides, we aren’t totally different, right?”

Sans’ expression softened and he relaxed in her hold. Water lapped at their bodies but neither paid any mind.

“No,” he finally said, “we aren’t.”

The warrior woman smirked. “Then can we finish what we started?” she asked, framing his face with her hands once more. Sans responded by doing the same to her, leaning up to close the distance with a kiss.

Lips against bone was different, but Undyne could get used to it. She pulled away with a grin, kissing him again at the top of his skull. 

“…so this is great, but, for real, though, we should probably call it a day. I heard my phone ring, like, six times.” 

“NNNNGGGAHHH, PAPYRUS THAT COCKBLOCK!!!!!”


	2. Chapter 2

**Summary for the Chapter:**

> Undyne steals Sans away for a mini vacation. Sans is Undyne's first.

**Notes for the Chapter:**

> I added a second, self-indulgent chapter. c:

Undyne stared into the mirror with a frown, irreparably frayed toothbrush grasped tight in a fist and a worn hairbrush in the other. Her hair felt weirdly soft as it draped over her shoulders, flowing in a way she wasn’t used to because she hadn’t groomed it in about six months. It was so much easier to just yank it back into a ponytail and forget about it. Her teeth felt strange, too, now a shiny white instead of the buttery, popcorn yellow shade she’d grown accustomed to. Why bother brushing your teeth when you were just gonna cram more food down your throat a few hours later, anyway?? 

Although, she would admit, she liked the way they glinted threateningly in the light. Maybe there was some merit to doing this twice a day, after all. She would just have to replace her toothbrush every week if she was going to rake it over her pointy teeth.

Double checking to make sure everything was in order, she left the bathroom and walked into her bedroom, instead. Tossing clothes violently out of her closet until she found a satisfactory date outfit, she tugged it on and grabbed an overnight bag with a fiery grin and determination in her eye.

Tonight, she was going to fuck a skeleton.

oOo

Sans was already waiting for her when she arrived in New Home, his usual tank top and shorts swapped out for trousers and a sweater, a drawstring bag resting on his back. When he looked up, the lights of his eyes brightened and he smiled. 

“Hey,” he greeted, straightening up. Undyne offered her own confident grin and embraced him tightly enough to lift him off the ground. “Your hair’s down today.”

“You’re wearing shoes today,” she replied, noticing a distinct lack of fuzzy, pink slippers on his feet. That must’ve been Papyrus’ doing, because he never took them off if he could help it. She set him down soon after and crossed her arms. “What’s shakin’, punk?”

“Nothing much,” he shrugged, reaching up for her hand. Webbed fingers threaded themselves between pale phalanges with practiced ease, and they started walking. “How is my _gill-friend_ doing today?”

Undyne was glad she couldn’t blush, because that stupid pun was actually really great and she hated it. It made her want to punch him in the face and kiss him fiercely at the same time. She adjusted her eyepatch and smirked down at the skeleton, a counter-measure prepared. 

“Great!” she said. “It’s better, now that I’m with the HOTTEST SKELETON IN THE WORLD!!!”

“Oh, geez,” he laughed, cheekbones stained a pale blue. Sans’ specialty was flustering his girlfriend with clever, flattering puns, and Undyne’s specialty was flustering her boyfriend with praise and loud compliments. It had become something of a contest between the two, to see who would give in and become undone first by the other’s overwhelming affection. Though she’d always argue otherwise, Undyne lost a majority of the time. 

“So,” the skeleton hummed, “where are we off to?”

“Someplace AWESOME!!” the Captain of the Royal Guard proclaimed, sweeping Sans into her arms and beginning to take off in a dead sprint. The male had long since grown accustomed to her antics and merely got comfy in her embrace, his arms winding loosely around her neck. 

Undyne bolted through the Underground, the world around them turning into a blur of noise and color, and it wasn’t for the first time that Sans found himself awed by her speed. She could probably run the length of the mountain in fifteen minutes if she really wanted to, and the shorter monster really wanted to see that one day. 

She ran for what felt like only a few minutes before coming to a stop in front of the MTT resort. Sans looked up at the building with interest.

“I didn’t think this kind of thing was your cup of tea,” he remarked, fingers brushing briefly across her neck as she set him down. She successfully withheld a shudder at the action. “You ever been here before?”

“Nah,” she replied, “but Mettaton owed me a favor so I decided to cash it in. Welcome to your new home for the next two nights, nerd!!” Her hand came down hard on his back and he stumbled, the blow softened by his bag. Sans didn’t mind, far used to her versions of physical affection by now, and shrugged leisurely.

“Okay, cool,” he said, “I’m not gonna complain about a free stay.” 

Though the words had left his mouth in an amicable manner, Sans really didn’t complain about anything except that which made him extremely uncomfortable, preferring to just tolerate whatever was going on so as to avoid conflict. Undyne hoped that wasn’t the case with the resort, but all she could do was trust that he would have a good time with her as they stepped through the doors.

They were immediately assaulted with shades of _pink_ and _chrome_ in all possible forms _everywhere_. Sans’ eye lights actually shrunk a little as he struggled to take in the lobby décor, his easy smile shifting into something closer to a grimace. Undyne felt the exact same way, honestly, but she couldn’t manipulate her pupils to regulate visual input and so was stuck taking in the place in all its glory. They’d both seemingly forgotten that Mettaton was not a humble monster, by any means. 

“Maybe the rest of the place isn’t like this,” Sans said, offering a placating smile to the taller fish woman. Undyne smiled back down at him, her confidence returning.

“Yeah!! The whole resort can’t look this dweeby!” she insisted, clutching his hand tighter and tugging him along. “Let’s go look around - BEFORE MY ONE GOOD EYE BLINDS ITSELF TO SPARE ME FROM THIS LOBBY!!!”

Sans chuckled, following dutifully along. They left the lobby and entered the hotel part of the resort, checking in under “Undyne the Undying.” The wait staff seemed to’ve been waiting for them, because the second the name left her lips, they were being led eagerly into the elevator and up to the top floor. Their key was rectangular and looked like the overgrown calculator himself, and Sans used it to open their room with barely restrained amusement. How someone could be this self-centered, he’d never know. 

It looked like someone had been told the phrase “honeymoon suite” and dialed it up ten notches. The bedroom was huge, the carpet a deep wine color with pink rose petals scattered amongst the floor. One wall consisted of floor-to-ceiling windows that bathed the room in natural lighting from the outside, with glass doors in the middle that led to a balcony (also decorated with rose petals), and the rest of the room was almost too furnished. Plush chairs were on either side of the even more plush California King bed, and a loveseat had been placed on the balcony with a bucket of champagne resting on ice beside it. Near the other end of the room sat a piano, brilliantly polished and lovingly taken care of. Undyne’s fingers itched to touch the keys, but she refrained for now. 

Neither monster had even looked at the bathroom yet, and they were already overwhelmed. Thankfully, nothing in the suite was Mettaton-shaped, sparing the captain of the Royal Guard from a potentially awkward attempt at seduction later on.

“Holy shit,” said Sans, shrugging off his drawstring bag and dumping it next to the door, “how big was that favor?”

“Hella big,” the warrior woman boasted, even though all she’d done was briefly mention that she wanted to take Sans someplace nice while visiting with Alphys, and the robot had practically tossed his resort brochure in her face. She thumbed the strap of her bag, feeling hopelessly underdressed in such a fancy place, and glanced at the skeleton as his gaze roamed their bedroom. “Whataya think, twerp? Too classy for ya?”

Sans could hear the anxiousness under the current of her words and turned, giving her a genuinely bright smile. “You kidding? This is the nicest place I’m ever going to occupy in my life. If it weren’t the middle of the afternoon, I’d be asleep in that bed right now.”

Undyne felt tension drain out of her she didn’t know she’d been harboring, and grinned brightly back at him. “Fuhuhu, don’t you DARE think we came out here just to take a nap, skeleton!!” she exclaimed, violently tossing her bag down next to his and grabbing his hand. Making sure they had their keys, she dragged him out of the room and back towards the elevator. “Right now, I’m STARVING!!! LET’S GO STUFF OUR FACES FOR ABSOLUTELY FREE!!!!!”

“I’m cool with that,” he replied, leaning into her side as they entered the elevator and gently pressing her hand to his teeth, the best he could do in terms of a kiss. It sent her heart in a tizzy and made her soul race with unbridled affection, and she almost dragged him back to the room right then and there. “Thanks for taking me out. I haven’t done anything like this in a while.”

“I know,” she huffed, freeing her hand so she could hook it around his neck and use the other to noogie him. “You don’t ever wanna leave the house unless I drag you out, punk!! ADVENTURE IS HEALTHY FOR YOUUUU!!!!”

“I don’t think chewing on rocks to see whose teeth are stronger is very healthy adventure.”

“YEAH, BECAUSE YOU FORFEITED AFTER GENTLY PRESSING A PEBBLE TO YOUR MOUTH!!!!”

Sans simply smiled and shrugged, stepping out of the elevator with her as the doors reopened and walking towards the restaurant. “I knew my teeth would’ve cracked if I tried. I could _feel it in my bones_ ,” he said, earning a groan and a shove from his girlfriend. 

“I changed my mind, punk; go home, this mini vacation is for me.”

“Hnn…nah,” he smirked, “you’d just get _bonely_ without me.”

God, she loved and hated the rush of heat his playful stare shot down to her loins, even though that pun was awful and she wanted to roll her good eye so far it fell out of the socket. Instead, she deliberately sped up her pace, forcing him into a jog to keep up with her, and entered the restaurant with a huff. 

Despite the gaudy decorating in half the resort, the restaurant didn’t look too bad. There were actually quite a few other monsters staying here, too. It was packed with friends and couples alike, all enjoying what appeared to be good food, and chatting vibrantly over soft music that played through the speakers. Sans was surprised to see Burgerpants acting as the host.

“I thought you worked in the –“ he began, his sentence trailing off as he stared into the lifeless eyes of the nineteen-year-old. “Er…never mind.”

“It’s okay, little buddy,” Burgerpants said, his smile strained and his tail twitching anxiously, “I was trained to do a lil’ bit of everything, y’see. Anyway, I won’t keep y’all waitin’, follow me.”

He turned, walking towards the back and leading them to the patio outside. The afternoon had begun to yield to dusk, the air comfortingly cool. The resort was built somewhat close to the wall of the mountain, so the crystals embedded in the stone bathed them in subtle blues and greens as they sat at their table. The feline monster handed them menus and promised to return shortly, leaving the duo alone. 

Undyne vividly recalled the lake she’d augmented for Sans as they sat, remembering the way the crystals had reflected off the water on his bones and made him glitter like a gemstone. He seemed to be recalling something similar, because he was staring at her in a way that was almost invadingly intimate, his smile one she hadn’t seen before. He seemed to realize he was blatantly ogling her and looked away, and if it weren’t for the crystals, she’d be able to clearly see his cheekbones flushing blue.

“I, uh…” he began, making Undyne’s stomach swirl. He didn’t hesitate to speak often. “I like it out here. The crystals remind me of the lake.”

Pleasantly surprised, Undyne grinned wide. “Me too, nerd!!” she exclaimed. “THAT WAS THE BEST FIRST DATE EVER!!!”

Sans’ smile blossomed into something bigger and he nodded, reaching his hand across the table. She didn’t hesitate to take it, enjoying the feel of his thumb as it brushed invisible patters into the back of her palm. 

God, she thought, his hands were so little. He was so cute she almost expected him to shit rainbows; but he didn’t shit, because he was a skeleton. She shook her head to dispel those thoughts, red hair following her movement, and she used her free hand to brush it out of her eye so she could read her menu. Sans found her concentration face cute. 

“HOLY SHIT!!!” she almost screamed, startling the skeleton. “THANK GOD WE’RE HERE FOR FREE, WHO EVEN WANTS A SIXTY GOLD, METTATON-SHAPED STEAK!?!?!”

Sans snorted, laughing into his free hand. “Oh god, Undyne, let’s get it.”

“What!?”

“Yeah,” he insisted, “let’s get it. We’re here for free, we may as well try some of this weird junk. Like a sampling party, or whatever.”

The fish woman’s eye widened. That was right; they could get whatever they wanted courtesy of the metal man himself! Her smile turned mischievous and Sans matched it, the two pouring over the menu to pick out the weirdest and priciest items. By the time Burgerpants returned to take their drink orders, they were ready to choose their entrées as well.

The feline seemed more than okay with the idea of them wasting half their food, and walked away to get their meals with a strangely blissful smile on his face. They reasoned, amusingly, that he must really hate Mettaton, if he wasn’t so much as blinking an eye at their order. Now that the excitement of getting weird food had dulled, though, Undyne found herself at a loss of words, and the two of them lapsed into silence. 

Man, she wished she were a more eloquent conversationalist, because a lot of their dates ended up as quiet as this. It wasn’t that they didn’t have things to talk about, it was just that they’d talked about it all before. The limited terrain of the Underground kind of did that to monsters; once you’ve seen it in its entirety, that was pretty much it, unless you found new ways to explore or manipulate it like the fish monster had. 

“Penny for your thoughts?” Sans said, startling her out of her musings. She shrugged, threading the fingers of her other hand through his and fiddling idly with the both of them. 

“Are you having a good time?” she asked, in a rare show of hesitance. She would’ve worried her lip between her teeth if she knew she wouldn’t accidentally bite herself. “I know I drag you around a lot, and you don’t always have fun –“

“Any day I’m out with you is a good one,” he interrupted, so sincere it made her pulse speed up. “Yeah, maybe I wasn’t too fond of that time you got me and Papyrus up at three in the morning for a snowball fight, but I slept like a champion afterwards, and you made Paps happy. I do have fun with you, even when I might not be at first.” The skeleton glanced towards the cavern wall, smiling at the crystals. “I was anxious, learning how to swim, until you showed me that it was actually really easy. Now it’s my favorite memory.”

The warrior woman looked at the table, suddenly finding the cloth fascinating. “Yeah, mine too, nerd,” she mumbled, squeezing his hands. “That was when we finally stopped being wimps and confessed our feelings!! It was HARDCORE!!”

“Can you believe that was seven months ago?” he asked. “It feels like forever.”

“The salt was made to last, I think,” Undyne remarked, nodding, “we could go swimming again sometime.”

Sans smirked and narrowed his eyes, almost purring, “Sure. I’d love to strip down and get wet with you again.”

Holy shit. Undyne’s pulse practically jackknifed, and she gripped his hands harder to keep her own from trembling. The skeleton took the action the wrong way, however, his expression shifting to something closer to embarrassment, and he pulled away.

“Heh, I’m sorry,” he said, laying his hands in his lap. He didn’t meet her gaze anymore. “That was way outta left field, wasn’t it? I overstepped a boundary –“

“N-no!” Undyne exclaimed. Sans looked at her oddly and she wanted to shove her fist in her mouth. “I mean…I didn’t mind…what you said. I actually…uh…” God, she was two seconds away from twiddling her thumbs like a dweeb. But she wasn’t a dweeb! THAT WAS FOR DWEEBS!!! “Tch, look, that was…kinda hot. Shut up, don’t give me that look.”

The male watched her stumble over her words, with a fondness that made her blood pump as if running a marathon, and it only pumped faster when his expression shifted into something less than innocent. He coughed into his hand, hiding his grin.

“Yeah?” he said. “I’ll keep that in mind, then…”

Undyne didn’t get a chance to reply, because Burgerpants, along with several other wait staff, returned with tray after tray of food. They pulled a second table up beside theirs to set the rest down, and the two of them realized that maybe they’d had a little too much fun taking advantage of their free stay.

Sans looked over it all with amusement, tapping his fingers against his drink. “I think I can eat, like, a tray and a half, if I push myself,” he said. “So you’ll have to take care of the other… six, or else we’re taking home about ten doggy bags worth of leftovers.” He stopped, blinking. “Actually, this is great. I can live off of Pap’s spaghetti and leftovers for the next week.”

Undyne cackled, more than ready to cram a bunch of fancy food down her throat, and slammed her fists on the table. Sans had to catch a tray to prevent it from toppling to the ground. “ONLY THE WEAK NEED TO BAG UP LEFTOVERS!!!”

“I’m weak, I’m so weak. The absolute weakest. In fact, let me call somebody over for some containers –“

“NNGGGAHHH, FEAST!!!” she roared, violently skewering a piece of Mettaton-shaped steak and leaning over the table to shove it into Sans’ mouth. The skeleton sputtered, his hand curling around her wrist to still her while he tried to calm his gag reflex, and bit down. If she hadn’t viciously assaulted his maw, the act of her feeding him could’ve been considered cute. “How is it, punk!?!?”

Sans leaned back in his chair, still chewing, and shrugged. Once he swallowed, he reached for his drink. “This information might be a little _hard to swallow_ -“

Undyne groaned. 

“- but I would pay sixty gold for that steak. That sentient toaster actually knows what he’s doing.” He seemed genuinely taken aback at the quality of the food. Undyne shoved another piece on her fork for herself and shoveled it down her throat, also pleasantly surprised.

“You’re right, it doesn’t taste like shit!” she stated, which made him laugh. “What’s so funny, dork!? WANT ME TO CRAM ANOTHER PIECE BETWEEN YOUR TEETH!?!”

“Heh, no,” he said, “you’re just cute.”

Undyne faltered, caught off-guard by his genuine compliment, and spared herself an embarrassing response by shoving more food in her mouth. 

The two of them then started eating in earnest, sampling the different entrées and swapping opinions over them. They took their time getting from tray to tray, sending back the foods neither liked and digging into the ones both enjoyed, and went slow so they could chat and savor each one.

“This one’s good,” Sans stated, having taken a bite of a quiche, “like, really good. Damn, I wonder if Paps would wanna try making this sometime.”

“Gimme some, punk,” the fish said. Sans obliged, cutting away a forkful and reaching across the table to feed her. Undyne figured now would be a good time to start trying to seduce him, and closed her lips slowly around his fork. She groaned at the taste – wow, that really was a good quiche – and felt Sans’ hand tremble as he gripped the silverware. Pulling away, she licked her lips and gave him a thumb up. “Nice!!”

“Y-yeah,” Sans mumbled, his arm retreating to his side. He looked at his fork with hesitance, then up at her. She flashed him an innocent grin and he smiled back. “We should get another to go.”

“We can share that one, too,” she said, in what she hoped was an alluring tone. Sans’ smile grew and he hummed in response, offering her another forkful. 

For a few minutes, a pattern emerged; Sans took a bite of the quiche, then fed Undyne a bite of the quiche, and alternated until the dish was nothing but crumbs. The skeleton leaned back in his chair and set the fork down, patting the space where his stomach would be underneath his sweater. 

“I’m full,” he declared. “Are you willing to be a weakling for my sake and box up the rest?”

The fish monster pretended to think on it, resting her chin on her fist and scrunching up her face to imitate hard concentration. Sans snorted at the sight. 

“I guess I can be a wimp just this once,” she finally decided, “BUT IF YOU TELL ANOTHER SOUL I’LL DENY IT AND THEN SUPLEX YOU TO DUST!!”

“I’m more than content to die in your arms, Undyne,” he chuckled, his words warm. Undyne hated how flustered that made her feel, and aggressively waved a waiter down for boxes.

oOo

With the night still young, the two of them deposited their boxes in the fridge of the room before exploring the rest of the resort. Sans suggested a visit to the gift shop and Undyne complied.

Almost everything was Mettaton-shaped, which amused the skeleton to no end, and Burgerpants was working the register when they entered.

“But, weren’t you just –“ Undyne began. 

“Yeah I was, lil’ buddy,” Burgerpants replied, his smile tight and tail twitching. “Then my coworker quit unexpectedly, so now I’m tacklin’ his shift. I ain’t surprised.”

“We’ll be quick,” Sans promised the cat monster. He simply nodded, closing his eyes for a quick nap behind the counter, and the male sympathized. He wandered through the aisles to look for a new action figure to bring home to Papyrus, and Undyne discreetly nudged Burgerpants awake again.

“Hey,” she whispered, “you got condoms in here?”

The 19-year-old blinked, unfazed, and nodded, pointing towards the back. “Through the door is the ‘adult’ section. Have fun,” he said, with no enthusiasm in his voice, and the redhead nodded her thanks. 

Double-checking to make sure Sans was still busy elsewhere, she slipped to the back and looked around. Unsurprisingly, one of the first things she spotted was an MTT dakimura, one side in his box form and the other in his EX body. Cringing, she moved past it, as well as the dildo shelf - how would you even fit a rectangular dildo inside of you, anyway!? – and quickly snagged what she needed.

When she reentered the safe part of the store, she almost ran right into Sans, whose gaze held nothing but mirth. 

“Hey,” he said, “how was the adult section? ‘Find what you want in there?”

“Keep looking at me like that AND I’LL BUY YOU A COPY OF HIS SEX TAPE FOR CHRISTMAS!!!” she roared, and he flashed his hands in surrender with a laugh. Embarrassed and frustrated, she brushed past him to check out, shoving the condoms in her pocket to prevent him from seeing them. “Ready to go, dork?!”

“Yeah,” Sans replied, paying for the figure he picked out for his brother. It was of Mettaton in his EX form, his hands on his hips and one leg straight up in the air. He cringed at it before it was placed in a bag, taking it from Burgerpants with a “thanks” before they left the shop. “I’ll never know what Pap sees in this guy.”

“Probably nothing but leg,” Undyne quipped. “Imagine what was going through Alphys’ mind when she built that body.”

The both of them shivered, sharing a look that promised neither would mention that again. They both respected their lizard friend, and they wanted to keep it that way.

“Where to now?” the skeleton asked, changing the subject. Undyne hummed, tapping her chin. 

“Why? So you can finish the rest of this date with the bed in our room?” she smirked. Sans grinned back.

“Hey, you knew when we started dating that you’d have to share my heart with Mistress Mattress,” he said, in the most serious tone he could muster, “don’t diss our love.”

“Fuhuhu, you nerd,” the warrior woman laughed, taking his hand. “I guess there’s nothing else to do down here. Let’s go upstairs, then.”

Sans nodded, following after her as they reentered the elevator and took it up to their floor. They entered the room, not bothering with the lights since the crystals outside illuminated it well enough, and he broke away to shove the action figure in the bottom of his bag, preferring not to look at it the rest of their stay. Undyne, uncharacteristically quiet, crossed her arms and felt flustered by the rose petals scattered all over the romantic-looking room. There were even some in the bed! And on the piano!! 

She walked up to the instrument and began brushing them away, angry at the buffoon dumb enough to place them there in the first place. Didn’t they know that they could easily get stuck between the keys!?

“Will you play?” Sans asked, startling her out of her thoughts. She turned to look at him and he repeated himself. “The piano…could you play a song for me?”

“Oh,” she blinked, adjusting her eyepatch. It wasn’t often that she performed in front of other people, but she found herself unable to deny him. “Uh, sure. What do you wanna hear?”

“Anything. Doesn’t matter,” he smiled, approaching her to stand next to it. He brushed his phalanges idly over a key, listening to the note ring briefly in the room, and Undyne suddenly had the most brilliant of ideas. It was brilliant by her standards, at least, and that’s what mattered!

Grabbing his hand, she sat down on the bench and pulled him into her lap, his back to her. Confused, Sans let her do it, looking up with an obvious question in his gaze. _What are you doing?_

Her expression was mischievous, but not malevolent, and she grinned. _Trust me._

“You can read music, right?” She asked him, knowing he sometimes played the trombone. The skeleton nodded and she was pleased. “Great! I’m gonna teach you how to play piano!”

“Oh,” he said, “but…my hands are probably too small.”

Undyne reached down and examined one of his hands, turning them over in her own. They were small, yes, but he could manage, she decided. 

“It’s fine, don’t worry about it!” she beamed. “We’ll start simple!”

“Shouldn’t I…” he began, fidgeting in her lap, “I dunno, are you sure I shouldn’t just sit beside you on the bench? This can’t be comfortable for you.”

“I’m sure,” she purred, kissing his cheek. “I don’t want you anywhere except against me.”

Sans flushed bright blue and shuddered, briefly clenching his fists. Undyne loved that she could make him just as embarrassed as he could her.

“Okay,” he murmured, nearly inaudible. Undyne reached out and moved his hands to the piano keys, briefly explaining the scale and which notes corresponded to it. Her hands covered his to act as a guide, gently pressing his fingers so he could, in turn, press the keys. 

Sans relaxed against her over time, quickly catching on to the lesson and repeating the simple tune she taught him with ease. Undyne rested her chin on his shoulder, murmuring occasional corrections into the side of his skull and telling him new notes to add onto the melody. 

Eventually she retracted one of her hands, then the other, leaving him to press into the keys by himself. Her arms wound around his waist as she listened to him tentatively play the song in its entirety, subtly pressing her fingers into his sweater. 

His breath hitched, a finger hitting the wrong note on the piano, and she chided him quietly. 

“Again,” she commanded, hugging him tighter to her chest. Sans took a moment to steady himself, then began to play once more. Again, she pressed her fingers against his body, causing him to mess up, and she teasingly reprimanded his performance. “C’mon, nerd, you’re better than this.”

“I-it’s, ah…” he mumbled, shivering. His cheekbones maintained a steady cyan throughout the session and his eyes were half-lidded. “It’s kinda hard to keep at it with your hands on me like that.”

“Do you not like it?” She asked, smirking. “Should I stop?”

“God, no,” he quickly replied. Embarrassed by his words, he couldn’t look her way. “It’s…nice.”

Undyne buried her face in his shoulder with a smug smile. “Play it again,” she demanded, gently squeezing his sides, and Sans barely managed to restrain a groan. 

With trembling fingers he restarted the song, coming to a stop when her hands began caressing his ribs through his shirt, and gasped. 

“Undyne…” he breathed, pulling his hands away to cover hers. In a display of dominance, she forced them back to the piano and pressed them firmly into the keys.

“I didn’t tell you to stop,” she growled. All pretenses of this being a lesson were thrown out the window when she moved the neck of his sweater aside to lick a stripe up his cervical vertebrae, drawing a low moan from the skeleton in her lap. He wanted nothing more than to run his hands over her body the way she was doing to him, but listened to her instead and obediently restarted the song. 

He messed up six more times after that, squirming and panting in her embrace as she pushed his shirt up to his chest and fondled his ribcage. She gently raked her nails over his sternum, relishing in the loud groan she pulled from the male.

“Undyne, _please,_ ” he begged, hands shaking so bad he couldn’t possibly hope to start again. She took pity on him and stopped her ministrations, watching him with one lidded eye.

“Please what?” she asked, her own breathing slightly labored. Watching him come undone under her hands was one of the most arousing things she’d ever seen. She wished she’d done this so much earlier.

“Please let me touch you,” he whined, looking pathetically up at her. There was something needy and primal in his gaze that she deeply enjoyed, quickly deciding to take pity on him. 

Rising from the piano bench, she hoisted Sans into her arms and carried him to the bed, setting him down and straddling him. The shorter male was on her in a second, tugging her jacket off and tossing it carelessly to the floor before his hands threaded themselves in her hair, pulling her down for a heavy kiss. It was more often for sentiment than anything else, but there was still passion in it as lips met teeth. 

Undyne resumed groping at the skeleton’s ribcage. Sans groaned into her mouth, running his hands roughly down her sides in _exactly_ the way she liked and causing her to press herself hard against him. He broke the kiss to bury his face in her neck, nipping gently at her skin. Her arms buckled, threatening to give out under his touch, and her mind started to cloud with pleasure. 

“Sans,” she moaned softly, making the skeleton sigh and grip her tighter in reaction. Slowly, she began grinding herself down against his pelvis, the both of them groaning at the sensation. 

“Haah,” Sans mumbled, staring up at her like she was the most precious thing in the world. It made her stomach swirl with butterflies. “F-fuck, you’re gorgeous…”

Undyne fisted the sheets on either side of him and closed her eye, trembling above him. Sans’ hands rested on her hips, bracing her and keeping the fish monster grounded.

“I,” she panted, “I want - can we…the whole way?”

The male nodded, his thumbs brushing over the exposed skin of her waist from where her shirt had ridden up. It was a soothing gesture, one she appreciated. 

“Whatever you want,” he promised. He sat up, coaxing her to switch positons, and straddled her waist with a calming smile. “Have you ever…?”

Undyne looked away, frowning. “Nah,” she confessed, the sheets suddenly very captivating.

That was the shitty part about being the youngest captain of the Royal Guard the Underground has seen in generations; she was placed on a pedestal that discouraged any potential partners, everyone assuming she was either too badass or too impersonal for simpler things like romance. She’d had boyfriends, sure, and a girlfriend at one point, that she’d fooled around with, but none of them had stayed with her long enough to get this close. 

None of them had been Sans, who laughed at the idea of imaginary pedestals and wanted to befriend someone regardless of status. Hell, it was the reason he was at least acquainted with more than half of the populace, from Woshua to King Asgore himself. It was a quality of his that she both envied and cherished. 

“Hey, that’s fine,” the skeleton insisted, drawing her out of her negative state of mind. He reached down and caressed her cheek soothingly. “We can go as slow as you want, and if you decide you want to stop, that’s fine, too.”

Undyne hesitated, her normally boastful personality tucking itself away and leaving behind a timid virgin. She reached for his hands, comforted in the way he automatically threaded their fingers together. 

“I want to keep going,” she said, quietly stubborn. “I want to give myself to you.”

Sans hummed, heartaching affection radiating from his soul, and he nodded. “I won’t make you regret it,” he promised, leaning down to capture another kiss. 

Undyne let him take the reins, lifting her arms when he tugged off her shirt and shivering as he trailed love bites down her throat, chest, and the expanse of her stomach. He made sure his pace was an easy one, slow enough for her to stop him if she wanted, but quick enough to keep her interest; her arousal quickly escalated from his ministrations, making the woman squirm under his touch. His fingers brushed against the waistband of her pants, looking at her silently for permission. She nodded, and once he started to tug them down, remembered herself and blinked. 

“W-wait,” she said, pleased to see Sans immediately back off and sit up. “There’s…something for you, in my back pocket…”

Sans raised a browbone, reaching into her pants and pulling out the box of condoms. “Oh,” he said, looking it over, “you got my size. Good guess.”

Undyne relaxed into the sheets, glad, and then immediately sat up again. 

“Wait, for real!?” She exclaimed, probably more excited than she should be. “You got a dick hidden in there!?” And, she thought, the XL pack was his right size!?

The skeleton snorted, covering his snickers in his elbow. “Sort of, yeah… I can conjure one.”

“W…well, let’s see it!” She said, crossing her arms in what she hoped looked more defiant and less insecure. “I’M NOT GONNA BE THE ONLY ONE NAKED WHILE WE TANGO IN THE SHEETS!!!”

God, Sans thought, she was fucking adorable. The lights in his sockets vanished, briefly leaving behind two dark pits, before his left eye bloomed its normal, royal blue. Undyne watched him shrug off his sweater while she shimmied the rest of the way out of her pants, wisps of blue traveling through his bones and down towards his pelvis. He smirked as she stared blatantly at his trousers, clearly waiting for him to discard them. 

Slowly, he pulled them off, leaving him in boxers and her in a bra and panties. A weak light was visible beneath the fabric and, to Undyne’s delight, took the shape of a cock. 

A thick cock. The warrior woman swallowed harshly, instinctively drawing her knees to her chest.

“I’d offer to…make it less intimidating,” Sans said, bashful, “but the size doesn’t change, sorry… if you want to stop –“

“I don’t,” she assured him, one golden eye staring determinedly into cobalt. “Just, y’know…take your time, or whatever…punk.”

“That’s my girl,” he praised softly, causing a torrent of butterflies to rush through the other monster’s system and pool in her nether region. He closed the distance between them and pulled her close for another kiss, fingers soothingly caressing her skin, and she relaxed against him enough to circle her arms around his neck. 

Carefully he eased Undyne to lie down, reaching underneath her and unclasping her bra. It came away easily and joined the growing pile of clothing on the floor. 

On a normal day, Undyne wasn’t shy about anything; she spoke her mind, did whatever she wanted, and didn’t think twice about her appearance. Lying here, now, though, beneath Sans, she was hyperaware of every scratch and scar marring her cyan skin, and felt uncomfortable under his gaze. Almost as if he could read her mind, Sans took her hand and pressed it against his ribcage, running it over nicks, hairline cracks, and many other imperfections in the bone. 

“I adore you. Every part of you,” he said, with such sincerity she almost choked up. Instead, she shot him a grin and nodded, curling her fingers in the spaces between his ribs and watching him shudder. He leaned down, kissing her forehead and tucking a strand of crimson hair behind her fin. 

“Undyne,” he said, “can I take off your underwear?”

It was another opportunity for her to change her mind, to put a stop to this and simply turn in for the night. The both of them were clearly fairly aroused, itching for contact, but Sans still reigned himself in and showed a restraint she didn’t know he possessed, all for the sake of her comfort. Even now, she could see the tremble in his frame as he leaned over her, his hands twitching and cock straining in his boxers. It was then and there she knew she wanted nothing more than to keep him by her side.

Nodding her consent, she framed his face in her hands. 

“…I love you.” 

The skeleton inhaled sharply, his iris flashing a brighter blue for a moment, before it welled with tears. Undyne, in that moment, felt panic surge through her. Was she not supposed to say that? Did he not feel the same? 

“I love you, too,” he said, voice shaky, and a grin so wide it seemed painful painted his face. “God, Undyne, I love you so much.”

She released a breath she didn’t know she’d been holding, her own tears close to falling. 

“Fu…fuhuhu,” she laughed, kissing him again. Her hands snaked down his body and rested on his boxers, tugging them down while he did the same to her panties, the last articles of clothing discarded to the floor. 

Sans’ dick glowed softly in the light, and Undyne looked at it in a mixture of curiosity and amusement. It didn’t have veins like one would on a flesh-and-blood monster, and if she hadn’t seen him conjure it, she would’ve sworn it was fake. Still, it was a very pretty shade of blue, and she somehow knew it would be pleasant to touch if she were to reach out and grasp it.

“…so, you’re gonna fuck me with a glowstick?” she teased. Sans rolled his iris and chuckled breathlessly, coaxing her legs apart so he could settle himself comfortably between them. 

“What can I say, you’re the _light of my life_ ,” he said. The fish monster groaned overdramatically.

“I don’t want to lose my virginity to a bunch of puns, dweeb.”

“Alright, that’s fair,” he reasoned. One hand kept him upright and the other ghosted over her body, taking his time to look over every single part of her as if committing this moment to memory. Hell, he probably was. Undyne was doing the very same to him, admiring the way the crystals outside illuminated him in the otherwise darkened room. 

His hand paused over her pubic mound, gaze flickering up to hers in a final question of consent. Undyne nodded, gasping afterwards at the sensation of bone caressing her pussy. 

Her legs twitched, hooking around Sans and crossing behind his spine. He leaned over until he was essentially parallel to the fish monster, toying with her labia with unbridled lust. Undyne puffed out a breath as he slipped a finger between them and brushed against her clit, circling the button in practiced motions, the slick sounds of her arousal more of a turn-on than it should’ve been.

“You okay?” the skeleton asked, his voice soft. “This feel alright?”

The woman responded by bucking into his hand, grunting softly and fisting the sheets. Sans must’ve really liked that response, because he cursed under his breath and leaned down to nip at the skin of her neck again. Undyne closed her eye and whined, a noise she didn’t think she could make, feeling his fingers prodding at the opening of her slit. She was dizzy from the stimulation, overwhelmed in the very best of ways, and weakly clutched at the back of his skull as he moved down to toy with her breasts. 

“I love the sounds you make,” he praised quietly, as if raising his voice would shatter the atmosphere, and fondled her cunt at a quicker pace. “You’re soaking wet, Undyne.”

“Nnngh…f-fuckin’…great,” the taller monster babbled, drunk on these new sensations. Sans cooed at her and chuckled, planting kisses down her torso that left her shivering and desperate for more.

“I’m gonna push a finger in,” he stated, beginning to nudge the digit past her virgin walls. The male groaned at the sight of her back arching off the sheets, getting worked up by just watching her alone, and relished in the feel of her hand tightly gripping the back of his head. “That’s it, baby.”

“S-Sans,” Undyne panted, unsure how to take the invasion. She trusted that he’d take care of her, but adjusting to all of this new stimulation at once was difficult. She found herself struggling to stay grounded, drowning in ecstasy. 

“I’m right here,” Sans promised, pressing his forehead against hers. Undyne opened her eye and stared hazily up into his blue iris, calmed by the color and the familiar pulsation of his magic. It brushed against the green of her own, making her feel safe as she surrendered herself to his care. 

“Are you okay?” he inquired, nearly a whisper. 

“Yes,” she murmured, breathless, and gripped his arms as he pumped his finger in and out of her at a steady pace. “H-hah…feels good…”

“Good,” he purred. “All I want to do is make you feel good…you have no idea how beautiful you look in this moment.”

Undyne couldn’t blush, but she could feel warmth blossom in her chest at his words. He thought she was beautiful. 

“I’m gonna push another finger in,” he said, “so I can stretch you out. I want this to be as painless as possible, okay?”

He inserted the second phalange when she nodded, scissoring her opening until he could comfortably add a third. To distract from the uncomfortable sensation, he curled them in an attempt to brush against her sweet spot. The skeleton laughed breathlessly when she cried out, thrusting hard up into his hand and breathing heavily. 

Gradually he loosened her up, withdrawing his fingers and sitting back when he deemed her adequately prepared (and when the last of his patience began to wane). The woman nearly growled at the loss, lifting her head to ask him why he stopped when she saw him unwrapping one of the condoms. Luck must’ve been on their side, because it wasn’t chrome or pink or any other stupidly vibrant color that reminded them of Mettaton, and Sans shot her an apologetic look.

“There’s, ah, no sexy way to put one of these on,” he admitted as she watched him, carefully rolling it over his cock before shifting back between her legs. “Are you ready, Undyne?”

For a split second, Undyne wasn’t ready. She was about to have sex for the first time, and while she knew, logically, it wasn’t as big of a deal others made it out to be, it was still an important experience. She briefly considered all the things that could go wrong – the condom breaking, not being able to climax, _pain_ \- and then she looked up at Sans, smiling at her with all the patience of a saint, and knew she’d be fine. 

“…yeah, nerd,” she said, clutching his shoulders. “… _bone me._ ”

Sans blinked slowly, absorbing her words, and puffed out a breath. It threatened to become a laugh but he managed to reign it in, exhaling harshly.

“You’re so fucking cute,” he mumbled, as if astonished by this knowledge, and buried his face in her neck. Undyne wrapped her arms around him, clutching his back as she felt the head of his cock pressing against her entrance.

She tensed reflexively and Sans paused, allowing her to calm down again before slowly beginning to push inside. Undyne wasn’t in pain, per se, but the sensation of feeling full was one she wasn’t used to. Fleetingly, she wondered if this was how Thanksgiving turkeys felt when you crammed them with stuffing, and almost laughed at the thought. Her legs instinctively spread further to accommodate him, hissing through her teeth as he finally buried himself to the hilt. 

The skeleton stopped, then, the only noise in the room being the sounds of their labored breathing, and he waited for her to get used to the feeling of him inside her. His arms trembled and his expression was slightly strained.

“D-damn,” he breathed, panting softly. Sans lowered himself onto his elbows, his hands fisting the sheets on either side of her head. “Y-you’re really tight…like…like “I do Kegels every Tuesday evening for an hour” tight…”

God, he was the epitome of romance. “…thanks?” she replied, unsure as to whether or not it was a compliment.

Sans chuckled breathlessly. “You doin’ okay?”

Undyne swallowed dryly and nodded. Her body trembled beneath his, trying to adjust.

“Good…tell me when I can move,” he said, stroking her cheek. She leaned into his hand, sighing, and focused on taking deep breaths.

“…’m okay,” she finally mumbled about a minute later. “Move, Sans.”

The skeleton straightened, pulling almost all the way out before shallowly thrusting back in again. Undyne grunted from the action, her fingers weaving themselves into his ribs, and stared at the ceiling as she tried to understand how she felt. 

It wasn’t bad, but she was almost positive it was supposed to feel better than… _oh._

Oh, there it was. It was like someone had flipped a switch in her pussy, the discomfort shifting almost instantaneously to mind-tingling pleasure, and she moaned low and loud. There was something to this sex thing after all! She couldn’t believe she was missing out! The woman missed the relief on Sans’ face at finally seeing her enjoy herself, choosing instead to close her eye and clutch him tighter. 

“H-hard,” she stammered, drunk on these new sensations, “Sans, harder, go harder –“

“I hear ya,” he promised, his hands curling around her hips. He used that as leverage, thrusting deeper inside her with a groan. “I h-hear ya loud and clear, _fuck…_ ”

His hips moved at a steady pace, getting faster at Undyne’s pleading, and the warrior woman thrust back up into him the best she could. Like his fingers, his cock brushed against a particularly pleasant bundle of nerves that left the redhead breathless and seeing stars, and she was whimpering and whining with abandon. 

“Sans, _yes_ , holy f-fuck!” she cried, practically yanking him down for a kiss. He reciprocated the best he could, their bodies jostling with every thrust. “Fuck m-me harder, Sans, haah…pl-please!”

Sans obeyed, moving with such force the headboard was beginning to smack against the wall, and muttered fragmented praises into her skin. 

“Hahh, fuck you’re so f-fucking gorgeous,” he panted, “can’t believe I get to touch you like this…can’t believe y-you’re actually mine, fuck I-I love you so much, Undyne…”

What felt like a hot coil slowly being pressurized at her core was now dangerously close to bursting. Undyne couldn’t believe how _amazing_ it felt to be pinned to the mattress, thrust into with enough force to bruise and clinging to warm, smooth bone like it would evaporate from her fingers if she let go. Sans was here, he was the one making her feel like she could fucking fly, and she wouldn’t have a single complaint if this is how she died. 

She loved him. She loved him so much it physically hurt to imagine life without him in it. 

“I love you,” she moaned into the side of his skull, “I l-love you, Sans, haah, fuck!”

The skeleton was practically rattling above her, picking up speed as he felt her beginning to get close. He reached between their bodies and found her clit, stroking it with vigor as he opened his jaw, brought it to the junction of her shoulder, and bit down. 

That seemed to do it for her. All at once, the coil inside Undyne snapped and she was cumming, she was _cumming she was cumming_ , and everything went white and she couldn’t breathe, fuck, she couldn’t breathe and she was having a god damn spiritual experience underneath him. Pleasure she didn’t even know was attainable was reverberating throughout the entirety of her being, and her soul felt like it was going to burst from the emotions consuming the monsteress. She was only vaguely aware of the fact that she’d screamed his name in climax, indicated by the soreness of her throat when she finally began to come back down. 

Her vision swam back into focus and the static faded away into real sound. Sensation returned, and she was absolutely slick with sweat, and Sans was panting harshly above her, and she felt him pull out with a shiver and collapse on the sheets next to her, and she was _exhausted_. Out of the corner of her eye, she saw Sans remove the condom and toss it in the trash, before rolling back over and draping his arm around her with an equally exhausted huff. His iris dimmed and his cock vanished, the normal, white pips of his eyes returning.

“Are you okay?” he puffed, reaching a shaky hand up to brush some hair away from her forehead. She mustered up the energy to roll over and face him. 

“Are you kidding?” she asked, grinning like an idiot. “I feel like I can suplex the Barrier!”

Sans’ face relaxed into something fond, and he closed his eyes. In the crystal light, she could see sweat running down his bones. “Good…” he mumbled, yawning. “I’m glad that your first time was good.”

Undyne shifted and pressed her body against his, entwining their legs and burying her face in his chest. He held her close, like she was a precious treasure he couldn’t bear to lose, and idly threaded his fingers through her hair.

“It was perfect, nerd,” she grinned, her eye drooping. “Can we do it again?”

Sans laughed weakly. “Yes, please,” he mumbled into her hair. “But I need to sleep first, heh…goodnight, Undyne.”

“Night…Love ya’, punk,” she yawned, and in seconds, she was unconscious.

oOo 

The captain of the Royal Guard normally prided herself at being up and about before dawn, but that wasn’t the case this time. When she cracked her eye open, the artificial sunlight of the Underground was shining angrily at her face, and all at once she became aware of how _sore_ she was. She groaned, rubbing the sleep from her eye and beginning to sit up. 

“G-ah, fuck!” Undyne winced, pain blossoming from her hips and between her legs as she became aware of the hand-shaped bruises marring the cobalt skin. Her shoulder burned as well, her hand brushing over the bite mark, and she reeked of sweat and sex. The mattress shifted next to her and she turned, looking into the lightless sockets of her skeletal boyfriend as he began to regain consciousness.

“Mmm…hey,” he mumbled, his voice low and rumbly from sleep. The sound went straight to her core, blossoming new arousal. He sat up and palmed his eyes, before turning to her with a tired, lazy smile and fuzzy lights that she also found hot as hell. “G’mornin’.”

“I think it’s the afternoon,” she corrected, turning to look at the clock on the nightstand. It was almost one. “Yeah, after noon. We slept like rocks!” Sans’ hand moved her hair aside, brushing over her back, and she happily leaned against it. He drew away and she turned, confused. 

“Sorry. I’ll hold you in a sec, but…” he said, bringing his hand to her back again and swiping at it. “You have rose petals stuck to your skin.”

Ah, that was right. There were petals scattered across the sheets when they fucked last night, no doubt now plastered to their bodies from the sweat they’d produced. She turned to look at him and broke into laughter; he had a whole cluster of them stuck in the spaces between his lumbar vertebrae, and several others sticking to his shoulder blades, patellas, and even his feet. He was not as amused as she.

“Ha ha,” he drawled, gesturing to himself. “Shut up and help your poor boyfriend. These are uncomfortable.”

The fish monster composed herself, snickering occasionally as she carefully pried the petals from his bones. Together they got the ones from between the tarsals of his feet, and she coaxed him to lie down so she could get the ones stuck in his spine. Her fingers brushed methodically over his vertebrae, eliciting a shudder from him as she extracted each bit. She smirked at his reaction and kissed his forehead. 

“We’re still naked,” she murmured, “did you wanna go again?”

Sans hummed, his fingers brushing against the black and blue hand-shaped bruises on her hips. “I don’t think I should,” he replied, actually looking a little guilty over what he’d done. His gaze lingered on her shoulder, where his teeth had punctured the skin and left a distinct mark. “I think I went a little overboard with your first time. I’m –“

“If the next word outta your mouth is “sorry,” I’m gonna cram all these petals back in your bones and then suck them out again,” she threatened, golden eye glinting mischievously as she bared her teeth at him. The skeleton actually looked very tempted by that prospect. “Listen to me, punk, that was the best night of my life next to becoming Captain of the Guard, got it? I don’t regret a damn second.”

She sat back, gesturing to the marks he left with pride.

“These are HELLA COOL!!!! I look like I got in a fight with a VAMPIRE! Or… something else with sharp teeth!!!”

Sans was staring at her fondly again, in that way that made her feel like the only person in the world, and she flusteredly looked away.

“S-so…don’t feel guilty or anything,” she finished, mumbling. Sans sat up and hugged her to his chest, pressing his teeth to her temple. 

“Okay,” he agreed quietly, and rubbed small circles in her back. “We should probably bathe. I love you, but, we both kinda reek.”

“Fuhuhu…together?” she offered, grinning at him. He returned the expression and nodded.

“Yeah,” he said, “together.”

**Author's Note:**

> I kinda imagined Undyne increasing the salinity a whole bunch so that Sans could float. Would that work in real life? I dunno. I was picturing the Great Salt Lake in my mind when I "scienced" it out at like 4am. 
> 
> Anyway, this is just self-indulgent, rare-pairing. I hope you like it as much as I liked writing it!


End file.
